A policeman who left his gun behind after searching a child's bedroom made a silly and dangerous mistake, but he should not be overly criticised, his chief constable said.

The Police Service of Northern Ireland officer is facing disciplinary action over the incident in north Belfast on Tuesday.

He and fellow officers raided the property in the Ardoyne area and arrested a 39-year-old man on suspicion of dissident republican activity.

But the firearm and ammunition was left behind in a child's bedroom, though there were no children in the house at the time.

Upon realising the weapon was missing, police returned to the house and retrieved it 20 minutes later.

Chief Constable Matt Baggott apologised for the incident, but he said if the officer was punished it would be a "proportionate" sanction.

"It is serious leaving a gun in a house, it is serious," he said. "But imagine how that officer felt when he got back in that van and suddenly thought 'anybody got my...' and suddenly realised he left it in that house. He wouldn't have done that deliberately, he may be careless.

"We hold people accountable for that but if we were starting to pillory and suspend everybody for example in the health service that made a mistake in the course of their career in nursing or doctors we wouldn't have many people left in health, if we did that in teaching we'd have nobody left.

"We take it seriously, it is a serious issue, but ultimately get it in context: that same officer I have no doubt has put his life on the line on dozens of occasions before, so he made a mistake, it was a silly mistake and dangerous one but it was a mistake."

He added: "There will be an investigation and I can't talk about what the punishment might be but it will be proportionate, it will take into account what happened and why it happened and it will take into account what the officer has delivered over the years."